1. Field of the Invention
This invention relates to a magnetic reproducing apparatus, and more particularly to a magnetic reproducing apparatus adapted for converting, e.g., a video signal to a digital signal to reproduce a signal recorded on a magnetic recording medium by making use of a so called partial response system.
2. Description of the Prior Art
Generally, in magnetic recording/reproduction, in order to compensate an amplitude distortion or a phase distortion, etc. resulting from the recording/reproducing characteristic with respect to a magnetic recording medium, an equalizer is used. In recent years, an adaptive equalizing system as used in communication has been adopted also in such magnetic recording/reproduction.
The adaptive equalizing has been conventionally developed as a technology to carry out high speed data transmission utilizing a telephone line. In the telephone line, the transmission path characteristic varies depending upon the connection state of the line. For this reason, since the transmission path characteristic can not be sufficiently corrected with a fixed equalizer, there occurs the necessity of adaptively adjusting the characteristic of the equalizer.
In such a communication system, there are methods in which a signal of which the waveform, etc. is known in advance is transmitted to examine the transmission path characteristic thereafter to transmit a necessary signal (automatic equalizing), and a method in which a signal itself desired to be transmitted is used to examine the transmission path characteristic (adaptive equalizing). In either case, the object of the equalizer is to automatically eliminate distortion from a received signal waveform which was distorted in passing through a transmission path to faithfully restore a transmit signal waveform.
In order to consider application to the magnetic recording/reproduction of adaptive equalizing as described above, such a digital VTR (Video Tape Recorder) adapted to convert a video signal to a digital signal to record the digital signal onto a magnetic tape (video tape) or reproducing it therefrom by using the so-called partial response system is assumed as a magnetic recording/reproducing apparatus. This partial response system is a system of shaping a spectrum of code positively utilizing interference between codes due to the transfer characteristic of the transmission path (or recording medium). For example, NRZI code, Interleaved NRZI code, and the like belong to the partial response. On the recording side, there is provided a so called precoder to convert input data to an intermediate series in order to avoid propagation of code errors at the time of reproduction (discrimination). An example of the configuration on the reproducing side in the case where the above-mentioned adaptive equalizing system is adopted for a digital VTR for carrying out magnetic recording/reproduction by making use of such a partial response system is shown in FIG. 6.
In FIG. 6, a magnetic signal recorded on a magnetic tape (not shown) is converted to an electric signal by using a magnetic head 101. The electric signal thus obtained is then amplified by means of a reproducing amplifier 102. The signal thus amplified is sent to a detection characteristic circuit 103. This detection characteristic circuit 103 has a detection characteristic (decoder characteristic) of the partial response, and a characteristic of (1+D) in the case of the class IV. An output signal from the detection characteristic circuit 103 is delivered to an equalizer 104 comprised of so called a FIR (Finite Impulse Response) or a transversal filter, at which adaptive equalizing processing is implemented to that output signal. The output signal thus processed is then delivered to a decoder 105, at which discrimination between "1" and "0" by level comparison (comparate), etc. is carried out. Thus, decoding of data series at the time of recording is conducted.
An output d from the decoder 105 is sent to an adder (error detector) 106, at which an output y from the equalizer 104 is subtracted from the output d. Thus, an error (residual) e is taken out. The error e thus obtained is sent to an adaptive control unit 107. To this adaptive control unit 107, an output x from the detection characteristic circuit 103 is delivered as a so called reference input. The adaptive control unit 107 adjusts the filter characteristic of the equalizer 104 so as to allow a signal power of the error (residual) to be minimum. In the case where a so called transversal filter is used as the equalizer 104, multiplication coefficients (tap coefficients) for every taps are adaptively modified (corrected) and updated. Thus, the characteristic of the transversal filter is adjusted so that a characteristic close to an inverse characteristic of the electromagnetic conversion characteristic in the magnetic recording/reproduction is provided.
An output from the decoder 105 is sent to a signal processing circuit 108, at which reproduction of a synchronous block and/or error correction, etc. are carried out. The signal thus processed is sent to a video signal processing circuit 109, at which the restoring of original image data is carried out. In addition, although not shown, output data from the signal processing circuit 108 is sent to an video signal processing circuit and a sub-code signal processing circuit, etc. In these circuits, respective processing is carried out.
Meanwhile, in digital VTRs, a quantity of data to be recorded and reproduced becomes extremely large. Particularly, in VTRs of HD (High Definition) TV signal, or the like, when a relative speed of the magnetic head and the tape is merely increased to carry out recording by using a lesser number of channels, there is a limit to the quantity of recordable and reproducible data. As a result, there are many instances where it is impossible to realize a desired system. In view of this, a technique is employed to increase the number of channels for recording and reproduction to realize a great increase in the quantity of recordable and reproducible data by using a magnetic head assembly of the multi-channel type (so called a multi-channel head).
However, the provision of individual adaptive equalizers for every respective channels of such a multi-channel head disadvantageously increases the circuit scale, resulting in increased cost,